Aspects of the present invention specify a pressure sensor that has a high integration density.
A pressure sensor has a substrate that is provided with conductor tracks for contact-connecting electrical components. The pressure sensor has a measuring element for recording and converting a mechanical measurement variable or an electrical intermediate variable into an electrical signal. The pressure sensor has a signal converter for further processing the electrical signal output by the measuring element. The signal converter is used, for example, to amplify the signal from the measuring transducer as well as to calculate temperature compensation and to standardize the measurement signal. The substrate of the pressure sensor forms, together with a first diaphragm, a first closed cavity which contains an inert filling medium, for example, oil.
At least one side of the measuring element of the pressure sensor, which comprises an active surface, has direct contact with the filling medium in the first cavity. The signal converter is preferably arranged on the substrate of the pressure sensor in the form of an unhoused chip (die) which comprises integrated circuits. In one preferred embodiment, one side of the signal converter, a silicon chip with integrated circuits or switching structures, is preferably in direct contact with the substrate of the pressure sensor. The signal converter therefore has considerably smaller dimensions than housed chips which are present in SMD form, for example. SMD chips are surrounded, for example, by a plastic or ceramic housing. A silicon chip with integrated conductor tracks is bonded to external contacts in the interior. The small dimensions of the silicon chip make it possible for the signal converter to be arranged in a depression of the substrate, for example.
In one preferred embodiment, the signal converter is arranged in a depression of the substrate.
In order to protect the signal converter, which comprises a silicon material with an integrated circuit, from mechanical damage and harmful environmental influences or corrosion, the signal converter is at least partially encased by an elastic material or preferably by a soft elastic material.
In another embodiment, the signal converter at least partially has direct contact with the filling medium.
In one preferred embodiment, the elastic material and the filling medium are the same substance.
In another embodiment, the first pressure sensor has an intermediate layer in sections between the first diaphragm and the substrate. The intermediate layer is preferably connected to the substrate by soldering or welding. The coefficient of thermal expansion of the intermediate layer is preferably matched to the coefficient of thermal expansion of the substrate. In one preferred embodiment, the intermediate layer is in the form of a ring to which the first diaphragm is applied. In one preferred embodiment, the intermediate layer contains Kovar.
Metal alloys which have a low coefficient of thermal expansion which is typically lower than the coefficient of most metals are referred to as Kovar. Such Kovar alloys have a coefficient of thermal expansion in the region of approximately 5 ppm/K. The different coefficients of thermal expansion of the substrate, of the diaphragm and of a process connection of the pressure sensor can be compensated for by the Kovar intermediate layer, thus resulting in tensions which are as low as possible in this case.
In one preferred embodiment, the diaphragm comprises a metal, in which case stainless steel is particularly suitable as the material for the diaphragm here.
In one preferred embodiment, the diaphragm has a structured surface which may comprise, for example, a concentrically spreading wavy pattern. The diaphragm is connected to the intermediate layer by soldering or welding, for example.
In order to measure the pressure of a medium relative to the ambient atmosphere in another embodiment, the rear side of the measuring element of the pressure sensor has direct contact with the atmosphere of the environment. For this purpose, the substrate of the pressure sensor has a cutout, for example, in the region of its rear side of the measuring element. The first cavity is preferably separated from the atmosphere of the environment by the measuring element.
In another embodiment, it is also possible for the pressure of the relative medium, for example, air, to be measured using a second measuring element.
In another embodiment, the pressure sensor is used to measure the differential pressure between two media applied to the pressure sensor. In this case, the substrate and a second metal diaphragm form a second closed cavity which contains an inert filling medium. The first cavity and the second cavity are preferably arranged on opposite sides of the substrate.
When measuring the differential pressure, the rear side of the measuring element has direct contact with the filling medium in the second cavity. The active side of the measuring element has direct contact with the filling medium in the first cavity.
In one preferred embodiment, the substrate comprises a ceramic which preferably comprises a plurality of layers. In this case, the ceramic may be, for example, an HTCC (high temperature cofired ceramic), for example, an aluminum oxide ceramic, or an LTCC (low temperature cofired ceramic), for example, a silicon oxide, an aluminum oxide or a lithium oxide ceramic.
The measuring element and the signal converter of the pressure sensor are preferably contact-connected via conductor tracks of the substrate. Bonding wires are preferably used to contact-connect the signal converter.
In order to contact-connect the pressure sensor to the outside, the conductor tracks of the substrate have at least one area which is arranged outside the first cavity and/or second cavity and can be contact-connected from the outside. As a result, the signal from the signal converter module can be directly output or processed further.
In order to fasten the pressure sensor to a media-filled tube whose pressure is intended to be determined, the pressure sensor has, at least on one side, a tubular connection. The medium to be measured has direct contact with the first or second diaphragm of the pressure sensor by the tubular connection. In one embodiment of the pressure sensor for measuring the differential pressure between two media, the pressure sensor has, on the sides of the cavities, a respective connection for fastening to the respective tube. The pressure sensor can also be fastened to the tube containing the medium to be measured, for example, using adapters between the pressure sensor connection and the tube.
The pressure sensor connection is preferably arranged on the outside of the first diaphragm and/or second diaphragm. In order to achieve a preferably gas-tight connection between the connection and the pressure sensor, the connection is sealed using an O-ring, for example.
In one embodiment, the connection may be in the form of a tube, a threaded connection, a grommet or else in the form of a so-called hose connection.
The pressure sensor is preferably suitable for measuring the absolute pressure or the relative pressure of a medium or for measuring the differential pressure between two media.
The following list of reference symbols may be used in conjunction with the drawings:    1 Substrate    2 Conductor track    3 Measuring element    4 Signal converter    5 First diaphragm    6 First cavity    7 Elastic material    8 Intermediate layer    9 Cutout    10 Second diaphragm    11 Second cavity    12 Contact area    13 Connection    14 First depression    15 Second depression